COMPONENTS OF ELECTRIC MOTORS

Introduction

Motor systems consume about 70% of all the electric energy used in the manufacturing sector of the United States. To date, most public and private programs to improve motor system energy efficiency have focused on the motor component. This is primarily due to the complexity associated with motor-driven equipment and the system as a whole. The electric motor itself, however, is only the core component of a much broader system of electrical and mechanical equipment that provides a service (e.g., refrigeration, compression, or fluid movement).

Numerous studies have shown that opportunities for efficiency improvement and performance optimization are actually much greater in the other components of the system—the controller, the mechanical system coupling, the driven equipment, and the interaction with the process operation. Despite these significant system-level opportunities, most efficiency improvement activities or programs have focused on the motor component or other individual components.

Direct-current (DC) motors are often used in variable speed applications. The DC motor can be designed to run at any speed within the limits imposed by centrifugal forces and commutation considerations. Many machine tools also use DC motors because of the ease with which speed can be adjusted.

All DC motors, other than the relatively small brushless types, use a commutator assembly on the rotor. This requires periodic maintenance and is partly responsible for the added cost of a DC motor when compared to an alternate-current (AC) squirrel-cage induction motor of the same power. The speed adjustment flexibility often justifies the extra cost.

Component Description

Field pole

Simply put, the interaction of two magnetic fields causes the rotation in a DC motor. The DC motor has field poles that are stationary and an armature that turns on bearings in the space between the field poles. A simple DC motor has two field poles: a north pole and a south pole. The magnetic lines of force extend across the opening between the poles from north to south. For larger or more complex motors there are one or more electromagnets. These electomagnets receive electricity from an outside power source and serve as the field structure.

Armature

When current goes through the armature, it becomes an electromagnet. The armature, cylindrical in shape, is linked to a drive shaft in order to drive the load. For the case of a small DC motor, the armature rotates in the magnetic field established by the poles, until the north and south poles of the magnets change location with respect to the armature. Once this happens, the current is reversed to switch the south and north poles of the armature.

Commutator

This component is found mainly in DC motors. Its purpose is to overturn the direction of the electric current in the armature. The commutator also aids in the transmission of current between the armature and the power source.

Rotor

Induction motor – Two types of rotors are used in induction motors: a squirrel-cage rotor or a wound rotor.

1.A squirrel-cage rotor consists of thick conducting bars embedded in parallel slots. These bars are short-circuited at both ends by means of short-circuiting rings.

2.A wound rotor has a three-phase, double-layer, distributed winding. It is wound for as many poles as the stator. The three phases are wyed internally and the other ends are connected to slip-rings mounted on a shaft with brushes resting on them.

Synchronous motor -- The main difference between the synchronous motor and the induction motor is that the rotor of the synchronous motor travels at the same speed as the rotating magnetic field. This is possible because the magnetic field of the rotor is no longer induced. The rotor either has permanent magnets or DC-excited currents, which are forced to lock into a certain position when confronted with another magnetic field.

Stator

Induction motor – The stator is made up of a number of stampings with slots to carry three-phase windings. It is wound for a definite number of poles. The windings are geometrically spaced 120 degrees apart.

Synchronous motor – The stator produces a rotating magnetic field that is proportional to the frequency supplied.

Electric motors are a major driving force in many industries. Their compact size and versatile application potentials make them a necessity. Motors are chosen many times because of the low vibration characteristics in driving equipment which has the potential to extend the life of the driven equipment. The higher rpm and small size of an electric motor will also make it a perfect fit for many applications.

Motors can be purchased for varying application areas, such as for operating in a potentially gaseous or explosive area. When purchasing a motor, be sure to check the classification of the area, you may have a motor that does not meet the classification it is presently in! For example, a relatively new line of motors is being manufactured with special external coatings that resist the elements. These were developed because of the chemical plant setting in which highly corrosive atmospheres were deteriorating steel housings. They are, for the most part, the same motors but have an epoxy or equivalent coating.

Often failures occur well before the expected design life span of the motor, and studies have shown that mechanical failures are the prime cause of premature electrical failures. Preventative maintenance takes steps to improve motor performance and to extend its life. Common preventative tasks include routine lubrication, allowing adequate ventilation, and ensuring the motor is not undergoing any type of unbalanced voltage situation.

The goal of predictive maintenance programs is to reduce maintenance costs by detecting problems early, which allows for better maintenance planning and fewer unexpected failures. Predictive maintenance programs for motors observe the temperatures, vibrations, and other data to determine a time for an overhaul or replacement of the motor.

Consult each motor’s instructions for maintenance guidelines. Motors are not all the same. Be careful not to think that what is good for one is good for all. For example, some motors require a periodic greasing of the bearings and some do not.

Motors, when properly selected and installed, are capable of operating for many years with a reasonably small amount of maintenance.

Before servicing a motor and motor-operated equipment, disconnect the power supply from motors and accessories. Use safe working practices during servicing of the equipment.

Clean motor surfaces and ventilation openings periodically, preferably with a vacuum cleaner. Heavy accumulations of dust and lint will result in overheating and premature motor failure.

Facility managers should inventory all motors in their facilities, beginning with the largest and those with the longest run-times. This inventory enables facility managers to make informed choices about replacement either before or after motor failure. Field testing motors prior to failure enables the facility manager to properly size replacements to match the actual driven load.

Reference :
Website
http://www.eere.energy.gov/femp/techassist/operations_maintenance/technologies/motors/types.cfm

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